1. Field of the Invention
The present invention relates to a method for the production of expanded polymeric materials, in particular for the production of polyurethane-based thermally insulating materials used in refrigerators.
2. Description of the Related Art
It is well known for expanded polymeric materials to be used in the production of insulating materials for refrigerators. A refrigerator (the term “refrigerator” being taken to mean any household electrical appliance for preserving foodstuffs, including freezers) essentially comprises a refrigeration unit located within/coupled with a container which is intended to keep foodstuffs at low temperature. The container is essentially constituted by a cabinet and a door, both of which are insulated. Over time, thermal insulation of the cabinet and the door has changed from the use of natural plant products such as cork to artificial products of mineral origin such as rock wool or glass fibre and finally to synthetic, expanded products. These latter products provide more consistent performance, better thermal insulation and furthermore contribute to forming the structure in combination with the inner shells (generally made of plastic) and the outer shells (generally made of sheet steel) of the cabinet and door.
The most commonly used material for thermal insulation of the cabinet and door is an appropriately formulated polyurethane resin (abbreviation: PU) with an added blowing agent. The polyurethane resin is in turn formed by the chemical reaction of two liquid components: one the polyol/polyether and the other an isocyanate, in the present case methylene diisocyanate, which is better known as MDI. The mixing/reaction of the two components (with an added blowing agent, for example, of the hydrocarbon type) occurs during the casting phase thereof within the air space formed by the outer shell and inner shell (liner) of the cabinet and/or door. Mixing the components gives rise to an exothermic chemical reaction, which ultimately results in the formation of the rigid expanded thermosetting polyurethane polymer. The blowing agent currently and most commonly used in Europe is a mixture of cyclopentane and isopentane, which is premixed with the polyol/polyether. Other blowing agents can, however, be used in refrigerator applications, for example R141b (India) or 245Fa (USA).
The exothermic reaction converts the blowing agent from the liquid phase (predispersed in the polyol) to the gaseous phase, so resulting in closed (non-intercommunicating) microcells within the polyurethane resin as it forms, which microcells impart elevated thermal insulation properties to the resin and simultaneously substantially reduce the apparent density of the “foam” formed (approx. 30 kg/m3).
Under the thrust of the blowing agent, the resin grows in volume (creating the foam) and so fills the entire air space of the structure (cabinet or door). Exactly the correct weight of resin must be dispensed so that no voids are formed or, conversely, so that the foam does not become “overpacked” in the air space.
While it is still in the liquid phase, the resin is capable of “wetting” the surfaces with which it comes into contact. The resin has substantial adhesiveness and this permits good adhesion between resin (foam) and the shells of the cabinet and the door. In this manner, an overall structure is obtained which is rigid and stable over time.
Another material used for thermal insulation is expanded polystyrene, which is known as EPS. Its insulating characteristics, while being good, do not match those of PU and are dependent on its apparent density; the best thermal insulation is achieved at an apparent density of the order of about 27-28 kg/m3. On the other hand, EPS is substantially lower in cost than PU.
The most commonly used blowing agent for EPS is pentane, which is added directly by the styrene resin producer in an amount of approximately 6% by weight or similar values. This resin, with the addition of the blowing agent, takes the form of solid microspheres (diameter of between, about 0.2 mm and about 0.7 mm). Given the volatility of the blowing agent, the material must be used (converted into its expanded state) within reasonably short periods or stored in tightly sealed containers. The expansion process takes place in two distinct phases, both on the moulder's premises: pre-expansion and final expansion/sintering. In the first phase, the microspheres or beads are heated with steam to a temperature such that the glass transition temperature (Tg), which is around 100° C., can be reached, thus enabling softening of the resin and expansion of the pentane predispersed therein, so forming beads with a diameter ranging from about 2 to about 4 mm. A fluidised-bed drying phase then follows, after which the beads are placed in suitable containers for the “maturation” phase. Expansion is around 95-97% of final expansion. The second expansion phase coincides with the actual moulding phase: the pre-expanded material is introduced into the mould that will impart the final shape to the part. The entire cavity of the mould is filled with the beads. After this operation, the final expansion and sintering phase is performed by introducing steam into the mould, which softens the beads and effects further and residual expansion of the pentane. The steam is introduced through the mould walls via numerous porous inserts that permit homogeneous diffusion of the steam within the entire cavity of the mould. This action results in the interstices between the individual spheroids being filled and simultaneously results in sintering thereof. A cooling phase follows before the parts are extracted from the mould. The processing range with regard to temperature has a lower limit at around 60° C. (the temperature at which expansion of the polystyrene begins) and an upper limit at a temperature of approximately 115-120° C. (above these latter values, the polystyrene begins to melt, causing the individual expanded spheroids to collapse). It is good practice to operate at between approximately 100 and 105° C. Parts with a good structure, but with limited surface hardness are obtained. A shell of plastics or metal can be introduced into the mould, the shell constituting one side of the finished part. During the final heating phase, the polystyrene beads adhere permanently to the inner side of the shell. The steam is introduced into the mould only from the side without the shell. It is much more difficult to introduce two shells into the mould, which would create a cavity as mentioned in the PU process for obtaining cabinets or doors. In practice, the presence of the vapour-impermeable shells makes introduction of the steam virtually impossible. The only option is to inject steam through the shell joints or through small holes made in the shells. The use of EPS as insulation and a structural component in refrigerators is thus greatly hindered by processing problems and by its lower insulating capacity. The use of EPS in refrigerators is thus conventionally restricted to premoulded fragments used as fillers in combination with PU. Another limitation on the use of EPS in refrigerators is that it is not completely impermeable to water vapour. Although the outer and inner walls of a refrigerator ought to provide a hermetic barrier to external gaseous agents, moist air can in fact pass through the joints in the shells (if they are not perfectly sealed) and, over time, penetrate within the EPS spheroids impairing their good thermal insulation properties.
A more widespread use is in thermal containers (for example, picnic coolers). In this case, the premoulded EPS part is mechanically assembled with two containment shells. As has been seen, the chemical reaction for the formation of PU, in conjunction with the expansion thereof, generates heat, while heat is required in order to achieve expansion of polystyrene. In the light of the above, the idea has arisen in the past of adding non-preexpanded polystyrene microspheres (with added pentane) to the polyurethane reaction mixture.
It has indeed been found that the heat of reaction of the PU is sufficient to soften and expand the polystyrene within the polyurethane composition, which is itself, expanded. Such a method is illustrated in U.S. Pat. No. 6,605,650 B1, with reference to the production of rigid structural articles.
Since the EPS microspheres that are formed are encapsulated by polyurethane resin they will be conveyed during the growing phase of the polyurethane foam and uniformly distributed. The disadvantage of the method described in the '650 patent resides in the fact that the ultimate object does not relate to maintaining or enhancing the thermal insulation properties of the polyurethane, and in the fact that the cost saving on the insulating material is very low given that the percentage by weight of polystyrene (relative to the weight of the sum of the polyurethane reagents) is no greater than 5%.
Furthermore, the method described in the '650 patent gives rise to a polyurethane matrix containing a plurality of microcavities lined with polystyrene, which arise from microspheres that have expanded and collapsed. This is deleterious with regard to achieving good thermal insulation properties and means that the technology described in the '650 patent cannot be applied to the production of insulating materials for refrigerators.